1. Field of the Invention
The invention relates to a current driving circuit which drives a current running through a load such as an organic electroluminescence device.
2. Description of the Related Art
Among loads which are required to drive a current running therethrough, a typical one is an organic electroluminescence device.
Though an organic electroluminescence device has to be developed in respect of enhancement in a brightness, a longer lifetime, a sealing structure of a module including an organic electroluminescence device and other parts, and so on, an organic electroluminescence device has many advantages: (a) it can be driven with a dc current at a low voltage; (b) it can accomplish a high brightness with a high efficiency; (c) it has a quicker response than a liquid crystal display device; (d) it has superior temperature characteristic at low temperatures; (e) it presents superior visibility; and (f) it emits a light from itself, and hence, it does not need to have a backlight device unlike a liquid crystal display device, and makes it possible to fabricate an image display device thinner than conventional ones when an image display device is designed to include an organic electroluminescence device. Because of the above-mentioned advantages, there is an eager need to be able to accomplish mass-production of an organic electroluminescence device.
As a circuit for driving an organic electroluminescence device applied to an image display device, an active matrix type driving circuit has been studied long, because it can present a high light-emitting efficiency and high image quality. The active matrix type driving circuit includes an amorphous or polysilicon thin film transistor (hereinafter, referred to simply as xe2x80x9cTFTxe2x80x9d) as an active device.
Japanese Unexamined Patent Publication No. 11-282419, for instance, has suggested an active matrix type current driving circuit including TFT for driving an organic electroluminescence device.
FIG. 1 is a circuit diagram of the suggested active matrix type current driving circuit.
The active matrix type current driving circuit is comprised of a first terminal 1 electrically connected to a voltage source, a second terminal 2 grounded, a signal line 3 through which a signal current runs, a selection line 4, an organic electroluminescence device 31 as a load through which a current required to be driven runs, a driving transistor 32, a transistor 33, a storage capacity 34, a first switch 35, and a second switch 36.
The organic electroluminescence device 31 is electrically connected at one end to the first terminal 1, and at the other end to the second terminal 2 through the driving transistor 32.
The driving transistor 32 controls a drive current in accordance with a voltage applied to a gate electrode thereof, and supplies the thus controlled drive current to the organic electroluminescence device 31.
The storage capacity 34 for keeping a constant voltage is electrically connected between a gate of the driving transistor 32 and the second terminal 2.
The second switch 36 is electrically connected at one of terminals to both the storage capacity 34 and the driving transistor 32, and at the other terminal to the transistor 33 which converts a current into a voltage.
The transistor 33 is designed to have the same polarity as that of the driving transistor 32. The transistor 33 has a drain and a gate both of which are electrically connected to each other, and hence, has a diode structure.
In addition, the transistor 33 and the driving transistor 32 cooperate with each other to define a current-mirror circuit through the second switch 36.
The transistor 33 is electrically connected to the signal line 3 through the first switch 35.
Each of the first and second switches 35 and 36 has a control terminal electrically connected to the selection line 4.
When a control signal is input into the selection line 4 to thereby turn both the first and second switches 35 and 36 on, a signal current running through the signal line 3 is input into the transistor 33 through the first switch 35, and is converted into a voltage in the transistor 33. The signal current further charges the storage capacity 34 through the second switch 36. Thus, a voltage associated with the signal current is stored in the storage capacity 34.
Since the transistor 33 and the driving transistor 32 define a current-mirror circuit through the second switch 36, a signal current running through the signal line 3 is supplied to the organic electroluminescence device 31 through the current-mirror circuit.
Even if a control signal is stopped to be input into the selection line 4, and accordingly, the first and second switches 35 and 36 are turned off, the voltage associated with a signal current running through the signal line 3 is stored in the storage capacity 34. Accordingly, the voltage stored in the storage capacity 34 is kept applied to a gate of the driving transistor 32. This ensures that a current which is the same as the signal current running through the signal line 3 is kept supplied to the organic electroluminescence device 31.
The conventional current driving circuit illustrated in FIG. 1 is accompanied with the following problems.
The first problem is that a signal current running is supplied to the organic electroluminescence device 31 from the signal line 3 with low accuracy.
The reason is as follows. A thin film transistor composed of an amorphous or polysilicon sometimes has variance in an order of hundred-millivolts with respect to a threshold voltage because of existence of grain boundary, unlike a semiconductor device composed of mono-crystal. As a result, in the current driving circuit illustrated in FIG. 1 including the transistor 33 and the driving transistor 32 both comprised of TFT, even if the transistors 33 and 32 are arranged adjacent to each other, it would be quite difficult or almost impossible to eliminate the above-mentioned variance in threshold voltages of the transistors 33 and 32, and match the transistors 33 and 32 with each other.
As a result, if the conventional current driving circuit illustrated in FIG. 1 were comprised of TFT, a resultant circuit could be fabricated only in a low fabrication yield and further in remarkably high fabrication costs.
In order to eliminate the above-mentioned variance in a threshold voltage of TFT, there has been suggested a method of processing signals not in an analog form, but in a digital form, for instance, in xe2x80x9ca patent has been issued to a circuit for enhancing accuracy in an organic electroluminescence panelxe2x80x9d, Nikkei Electronics, Apr. 24, 2000, No. 768.
However, the suggested circuit is unavoidably complex in a structure and large in a scale, resulting in an increase in fabrication costs.
The second problem is that the conventional current driving circuit illustrated in FIG. 1 consumes much electric power.
The reason is that though the signal current having run through the signal line 3 is supplied to the transistor 33 defining a current-mirror circuit together with the driving transistor 32, the current having run through the transistor 33 does not run directly through the organic electroluminescence device 31.
Japanese Patent No. 2953465 has suggested a current driving circuit including an input terminal, a first transistor having a drain electrically connected to the input terminal and a source grounded, a switching transistor electrically connected between a gate and a drain of the first transistor, a control terminal electrically connected to a gate of the switching transistor and receiving a signal in accordance with which the switching transistor is turned on or off, a second transistor having a gate electrically connected to the switching transistor and a source grounded, and defining a current-mirror circuit together with the first transistor, and a capacity device electrically connected at one of electrodes thereof to a gate of the second transistor, and grounded at the other electrode.
Japanese Unexamined Patent Publication No. 11-24606 has suggested a display device including a substrate, a plurality of scanning lines formed on the substrate, a plurality of data lines extending in a direction perpendicular to a direction in which the scanning lines extend, a plurality of common power-feeding lines extending in parallel with the data lines, a plurality of pixels defined by the data lines and the scanning lines in a matrix. Each of the pixels includes a first thin film transistor having a first gate electrode to which a scanning signal is transmitted through the scanning lines, a storage capacity storing therein image signals supplied from the data lines through the first thin film transistor, a second thin film transistor having a second gate electrode to which the image signals stored in the storage capacity are transmitted, and an organic semiconductor film which emits a light by a driving current running through a pixel electrode and an opposing electrode when the pixel electrode becomes electrically connected to the common power-feeding lines through the second thin film transistor. At opposite sides of each of the common power-feeding lines are arranged pixels through which the driving current runs from or to the common power-feeding line. The data lines extend at an opposite side of the common power-feeding line about the pixels.
Another active matrix type driving circuit has been suggested by K. Miyake et al., xe2x80x9cCurrent-Writing Active-Matrix Driving Circuit for Organic Light Emitting Diode Displayxe2x80x9d, Electronics Society Conference of Electronics Information Communication Institute, 2000, C-9-5, pp. 50. The suggested driving circuit is designed to include an amorphous silicon thin film transistor and have a function of canceling variance and/or shift in a threshold voltage in an organic light-emitting diode or a thin film transistor.
There is also suggested a polysilicon TFT display by M. Kimura et al. in xe2x80x9cLow-Temperature Poly-Si TFT Display using Light-Emitting-Polymerxe2x80x9d, AM-LCD 2000, AM3-1, pp. 245-248. The suggested low-temperature polysilicon thin film transistor (LT p-Si TFT) light-emitting-polymer displays (LEPDs) have the potential to be thin, compact, light weight, low cost, large and robust, as well as high resolution, low power consumption, a wide viewing angle, and a fast response. The advantages are achieved by combining the properties of both LT p-Si TFTs and LEPDs. Since a relatively large area in a pixel is available for TFTs, there is flexibility to choose a driving method.
However, the above-mentioned problems remain unsolved even in the above-mentioned disclosures.
In view of the above-mentioned problems in the conventional current driving circuits, it is an object of the present invention to provide a current driving circuit which can be fabricated in lower costs and consumes less electric power than conventional ones.
There is provided a current driving circuit including (a) a first terminal electrically connected to a voltage source, (b) a second terminal grounded, (c) a signal line through which a signal current runs, (d) a first switch, (e) a second switch electrically connected to the signal line and further electrically connected in series to the first switch, (f) a third switch electrically connected to the first terminal, (g) a memory stage which converts the signal current into a voltage and stores the voltage therein, (h) a driving transistor, (i) a load electrically connected between a source of the driving transistor and the second terminal, and (j) a selection line electrically connected to the first switch, the second switch and the third switch, wherein the signal line is electrically connected to a gate of the driving transistor through the first and second switches, the memory stage is electrically connected between a gate of the driving transistor and the second terminal, the first switch is electrically connected between a drain and a gate of the driving transistor, and the driving transistor has a drain which is electrically connected to the signal line through the second switch and further to the first terminal through the third switch.
It is preferable the first and second switches are turned on and the third switch is turned off when the selection line is in one of high and low levels, and the first and second switches are turned off and the third switch is turned on when the selection line is in the other level.
For instance, the load is comprised of an organic electroluminescence device.
There is further provided a current driving circuit including (a) a first terminal electrically connected to a voltage source, (b) a second terminal grounded, (c) a signal line through which a signal current runs, (d) a first switch transistor, (e) a second switch transistor, (f) a third switch transistor, (g) a storage capacity which converts the signal current into a voltage and stores the voltage therein, (h) a driving transistor, (i) a load electrically connected between a source of the driving transistor and the second terminal, and (j) a selection line electrically connected to gates of the first to third switch transistors, wherein the storage capacity is electrically connected between a gate of the driving transistor and the second terminal, the first and second switch transistors are electrically connected in series to each other between the signal line and the driving transistor, a connection point through which the first and second switch transistors are electrically connected to each other is electrically connected to a drain of the driving transistor, and the driving transistor has a drain electrically connected to the first terminal through the third switch transistor.
For instance, the first and second switch transistors may have the same polarity as a polarity of the driving transistor. As an alternative, the first and second switch transistors may have a polarity opposite to a polarity of the driving transistor.
For instance, the third switch transistor may have a polarity opposite to a polarity of the driving transistor, the first switch transistor and the second switch transistor. As an alternative, the third switch transistor may have a polarity which is identical with a polarity of the driving transistor, and is opposite to a polarity of the first and second switch transistors.
There is still further provided a current driving circuit including (a) a first terminal electrically connected to a voltage source, (b) a second terminal grounded, (c) a signal line through which a signal current runs, (d) a first switch transistor, (e) a second switch transistor, (f) a third switch transistor, (g) a storage capacity which converts the signal current into a voltage and stores the voltage therein, (h) a driving transistor, (i) a load electrically connected between a source of the driving transistor and the second terminal, and (j) a selection line electrically connected to gates of the first to third switch transistors, wherein the storage capacity is electrically connected between a gate of the driving transistor and the second terminal, the second switch transistor has a source electrically connected to a connection point through which a drain of the driving transistor and a drain of the third switch transistor are electrically connected to each other, the first and second switch transistors have drains electrically connected to the signal line, the first switch transistor has a source electrically connected to a connection point through which a gate of the driving transistor and the storage capacity are electrically connected to each other, and the driving transistor has a drain electrically connected to the first terminal through the third switch transistor.
There is yet further provided a current driving circuit including (a) a first terminal electrically connected to a voltage source, (b) a second terminal grounded, (c) a signal line through which a signal current runs, (d) a first switch transistor, (e) a second switch transistor, (f) a third switch transistor, (g) a fourth transistor, (h) a storage capacity which converts the signal current into a voltage and stores the voltage therein, (i) a driving transistor, (j) a load electrically connected between a source of the driving transistor and the second terminal, and (k) a selection line electrically connected to gates of the first to third switch transistors, wherein the storage capacity is electrically connected between a gate of the driving transistor and the second terminal, the second switch transistor has a source electrically connected to a connection point through which a drain of the driving transistor and a drain of the third switch transistor are electrically connected to each other, the second switch transistor has a drain electrically connected to the signal line, the fourth transistor is electrically connected between a drain of the first switch transistor and a drain of the second switch transistor, the fourth transistor has the same polarity as a polarity of the first and second switch transistors, the fourth transistor has a drain and a gate both of which are electrically connected to a connection point through which the second switch transistor and the signal line are electrically connected to each other, and a source electrically connected to a drain of the first switch transistor, the first switch transistor has a source electrically connected to a connection point through which a gate of the driving transistor and the storage capacity are electrically connected to each other, and the driving transistor has a drain electrically connected to the first terminal through the third switch transistor.
There is still yet further provided a current driving circuit including (a) a first terminal electrically connected to a voltage source, (b) a second terminal grounded, (c) a signal line through which a signal current runs, (d) a first switch transistor, (e) a second switch transistor, (f) a third switch transistor, (g) a fourth transistor, (h) a storage capacity which converts the signal current into a voltage and stores the voltage therein, (i) a driving transistor, (j) a load electrically connected between a source of the driving transistor and the second terminal, and (k) a selection line electrically connected to gates of the first to third switch transistors, wherein the storage capacity is electrically connected between a gate of the driving transistor and the second terminal, the second switch transistor has a source electrically connected to a connection point through which a drain of the driving transistor and a drain of the third switch transistor are electrically connected to each other, the first and second switch transistors have drains electrically connected to the signal line, the fourth transistor has a source electrically connected to a connection point through which the storage capacity and a gate of the driving transistor are electrically connected to each other, and a drain electrically connected to the signal line, the first switch transistor has a source electrically connected to a gate of the fourth transistor, and the driving transistor has a drain electrically connected to the first terminal through the third switch transistor.
There is further provided a current driving circuit including (a) a first terminal electrically connected to a voltage source, (b) a second terminal grounded, (c) a signal line through which a signal current runs, (d) a first switch transistor, (e) a second switch transistor, (f) a third switch transistor, (g) a fourth transistor, (h) a first storage capacity which converts the signal current into a voltage and stores the voltage therein, (i) a first storage capacity which converts the signal current into a voltage and stores the voltage therein, (j) a driving transistor, (k) a load electrically connected between a source of the driving transistor and the second terminal, and (l) a selection line electrically connected to gates of the first to third switch transistors, wherein the first storage capacity is electrically connected between a gate of the driving transistor and the second terminal, the first switch transistor is electrically connected between a drain and a gate of the driving transistor, the fourth transistor has a drain electrically connected to the signal line and a source electrically connected to a drain of the driving transistor, the second switch transistor is electrically connected between a drain and a gate of the fourth transistor, the second storage capacity is electrically connected between a gate of the fourth transistor and a gate of the driving transistor, and the driving transistor has a drain electrically connected to the first terminal through both the third switch transistor and the fourth transistor.
There is further provided an image display device including such a current driving circuit as mentioned above.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
The current driving circuit in accordance with the present invention is designed to include the storage capacity between a gate of the driving transistor and the second terminal. This ensures that a signal current supplied from the signal line is stored in the storage capacity, and further is converted into a signal voltage in the storage capacity, when the selection line is in a high level, and that even if the selection line is turned at a low level and hence the signal current is not supplied to the storage capacity, the driving transistor is driven by the signal voltage stored in the storage capacity, and accordingly, a current is supplied to the load such as an organic electroluminescence device.
Thus, the current driving circuit in accordance with the present invention is unlikely to be influenced by variance which would be generated in a process of fabricating a semiconductor device, and is able to enhance a fabrication yield of fabrication a semiconductor device.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.